Blood flows inside the blood vessels while keeping its fluidity. However, once the bleeding occurs, coagulation factors existing in the blood plasma and in the blood platelets are activated in a chain reaction. Fibrinogen contained in the blood plasma is transformed into fibrin and the precipitation of the fibrin leads to the hemostasis.
Such blood coagulability includes endogenous coagulability which stops the bleeding within a tissue and exogenous coagulability which stops the bleeding caused by an external injury, etc. Measurement items regarding the blood coagulability include the prothrombin time (PT) as an exogenous blood coagulation reaction test, the activated partial thromboplastin time (APTT) as an endogenous blood coagulation reaction test, the fibrinogen level (Fbg), etc. All these items are measured by detecting the precipitation of fibrin, caused by adding a reagent for starting the blood coagulation, by an optical, physical or electrical technique.
In a known method employing optical means, the time in which fibrin starts precipitating is calculated by irradiating the reaction solution with light and detecting the precipitation of the fibrin in the reaction solution as chronological intensity variation of scattered light or transmitted light.
When such an optical technique is used, effects of interfering substances (hemoglobin, bilirubin, chyle, etc.) in the sample are nonnegligible. Therefore, a lot of efforts have been made to figure out the way to eliminate the effects of the interfering substances. In a technique described in Patent Literature 1, for example, the reaction solution is irradiated with multiple-wavelength light. Data corresponding to a wavelength at which the effects of the interfering substances are weak is selected depending on the specimen (target of the measurement) and the selected data is used for the calculation of the coagulation time.
In a technique described in Patent Literature 2, a plurality of light scattering intensities are measured at different angles with respect to the optical axis of a single laser light source.